1. Field of the Invention
The present invention relates to a paper feeding device for separately transporting paper sheet by sheet from a paper feed tray in an image forming apparatus such as a copier, a facsimile, or a printer.
2. Description of the Related Art
The following technique as shown in FIGS. 26 and 27 has been known for use in such a paper feeder.
FIGS. 26 and 27 are explanatory illustrations of a conventional paper feeder which feeds paper from a paper feed tray.
In FIG. 26, a plurality of paper feed trays 01 are vertically stacked on top of each other. Each paper feed tray 01 is provided with a paper loaded plate 02 for supporting, in a raisable manner, that part of a lower surface of the paper piled in the paper feed tray which is close to a paper feed unit, paper side edge positioning members 03 for positioning the side edges of the paper housed in the tray, and a paper front edge support wall 04 for positioning the front edge of the paper in the transportation direction thereof.
A paper feed unit 06 is disposed in the direction perpendicular to the sheet of FIG. 26 along the side of each paper feed tray 01 close to the paper feed unit (i.e., the right side portion of the paper feed tray). The paper feed unit 06 comprises a pickup roller 07 for feeding the paper stored in the paper feed tray 01, a pivotal arm 08 for supporting the pickup roller 07, a tension spring 010 which pulls the pivotal arm 08 such that it downwardly presses the pickup roller 07, and a separately transporting roller 011 which comprises a feed roller 011a and a retard roller 011b which separately transport the received paper sheet by sheet.
The separately transporting roller 011 is disposed in front of the paper feed tray 01 in the paper feeding direction, and the pickup roller 07 is disposed above the paper feed tray 01. The pickup roller 07, the pivotal arm 08 for supporting the pickup roller 07, and a gear 12 for transmitting torque to the pickup roller 07 are disposed between the lower surface of an upper paper feed tray 01 and the upper surface of the paper loaded in a lower paper feed tray 01.
In FIG. 27, the pivotal arm 08 is provided with a supported pin 08a and a light shielding section 08b for detecting the position of the supported pin.
An operating link rod 014 and a spring link rod 015, both of which extend downwardly, are fixedly attached to a rotary shaft 013 rotatably supported by a frame (not shown). The lower end of the operating link rod 014 is joined to a leading end of an extendable rod 016a of a solenoid 16 so as to be relatively movable in a vertical direction as well as to be relatively rotatable.
The lower end of the spring link rod 015 is joined to a tension spring 017. When the lower end of the spring link rod 015 is downwardly pulled by means of the tension spring 017, the rotary shaft 013 and the operating link rod 014 rotate, which in turn results in the extendable rod 016a being extended.
When the extendable rod 016a contracts as a result of the solenoid 016 being turned on, the operating link rod 014, the rotary shaft 013, and the spring link rod 015 rotate.
A pin rasing member 013a is attached to the rotary shaft 013. When the solenoid 016 is in an OFF state, the rotary shaft 013 rotates via the spring joint rod 015 pulled by the tension spring 017, whereby the pin raising member 013a is retained in an elevated position. The pin raising member 013a, which is retained in the elevated position while the solenoid 016 is in the OFF state, raises the supported pin 08a. While the supported pin 08a is held in the raised condition, the pivotal arm 08 is also retained in an elevated position.
A position sensor 018 is fixedly supported by a frame (not shown) and comprises a light emitting sensor 018a and a light receiving sensor 018b which are spaced apart from each other. While the light shielding section 08b, which vertically moves in conjunction with the vertical movement of the pivotal arm 08, is in an elevated position, it is sandwiched between the light emitting sensor 018a and the light receiving sensor 018b.
A retard roller support frame 021 for supporting the retard roller 011b is supported by a support shaft 022 so as to pivot thereon. The retard roller support frame 021 is retained by a coiled tension spring 023 in such a position as to press the retard roller 011b against the feed roller 011a.
A retard roller support shaft 024 rotatably supported by the retard roller support frame 021 rotatably supports the retard roller 011b via a torque limiter 025. A gear 026 is attached to the retard roller support shaft 024.
A frame F supports a retard roller drive gear 027 which rotates around the support shaft 022.
When the retard roller drive gear 027 rotates, the gear 026 and the retard roller support shaft 024 are rotatably actuated. Resultant torque is transmitted to the retard roller 011b via the torque limiter 025.
The rotation direction of the retard roller 011b is set to be opposite to the paper feeding direction.
FIG. 28 is a diagrammatic representation for explaining the principle operation of the traditional paper feeder shown in FIGS. 26 and 27. With reference to FIG. 28, an explanation will now be given of the case where no paper or only one sheet of paper is pinched by a nipping section (that is, a contact area) between the feed roller 011a and the retard roller 011b. When the feed roller 011a rotates so as to transport the paper from left to right in FIG. 28, the retard roller 011b receives force in the paper feeding direction, from the feed roller 011a or the paper at the nipping section. At this time, the retard roller 011b rotates in a following manner in the direction opposite to the retard roller support shaft 024 that rotates in the direction opposite to the paper feeding direction (i.e., the retard roller 011b rotates in the direction in which the paper is fed). Arrows P1 to P3 shown in FIG. 28 designate the following forces:
P1: a tensile force of the tension spring 023, PA1 P2: an upward force which the gear 026 receives from the retard roller drive gear 027 in a meshed section between them in reaction to the force transmitted by the torque limiter 025, and PA1 P3: a pulling force which the retard roller 11b receives from the feed roller 011a by way of the nipping section in reaction to the force transmitted by the torque limiter 025.
Based on the assumption that distances between the center of the support shaft 022 and action lines of the forces P1, P2, and P3 are L1, L2, and L3, respectively, the sum of angular moments of the forces P1, P2, and P3 in the case of the feed roller shown in FIG. 28 can be expressed by the following expression: EQU P1.times.L1+P2.times.L2+P3.times.L3.
Provided that a force (i.e., a retarding pressure) by which the retard roller 011b is pressed against the feed roller 011a by means of the angular moments is P, and that the distance between the action line of the retarding pressure P and the center of the support shaft 22 is L, the retarding pressure P can be expressed by the following expression. EQU P=(P1.times.L1+P2.times.L2+P3.times.L3)/L
The paper transported between the feed roller 11a and 11b is separated sheet by sheet and fed on the basis of the retarding pressure P.
Problems to be Solved by the Invention!
With reference to FIGS. 29 and 30, the operation of the paper feed unit carried out when the retard roller 011b is reversely rotated so as to return the paper to the paper feed tray 01 will now be described.
Usually, the top few sheets of the paper loaded in the paper feed tray 01 are transported to the nipping section between the feed roller 011a and the retard roller 011b by means of the pickup roller 07.
Assume that two sheets of paper are introduced into the nipping section, as shown in FIG. 29A. Even if a lower sheet of paper tries to exceed the nipping section and advance further together with an upper sheet of paper currently being fed, the lower sheet of paper is returned to the nipping section by the retard roller 011b which reversely rotates, as shown in FIG. 29B. If the retard roller 011b is not reversely rotated, the lower sheet of paper remains in a stopped state at the position to which the lower sheet of paper advanced, as shown in FIG. 29C. If the stop position reaches a transport roller disposed in a downstream direction, the two sheets of paper are transported while overlapping with each other.
Assume that three sheets of paper are introduced into the nipping section, as shown in FIG. 30A. If the retard roller 011b is reversely rotated, it is possible to arrange the paper by returning the lowermost sheet of paper to the nipping section first and, subsequently, the second sheet of paper to the nipping section, as shown in FIG. 30B. If the retard roller 011b is not reversely rotated, the lowermost sheet of paper and the middle sheet of paper are separated from each other, as a result of which the top two sheets of paper may be fed together in an overlapped manner, as shown in FIG. 30C.
Overlapped sheets of paper are more frequently transported when there is a high probability of the top several sheets of paper being introduced into the nipping section by the pickup roller 70, that is, in the case where the paper is fed and introduced into the nipping section at high speed with an increased inertial force. The reverse rotation of the retard roller 011b contributes to the prevention of transportation of overlapped sheets of paper at a relatively high speed. If the paper is not fed at such a speed, the use of the drive force transmission means for reverse drive purposes adds to the cost of the paper feeder. Further, if the retard roller 011b is reversely rotated, load torque and power consumption increase, which in turn leads to increased noise.